1. Field of the Invention
The present invention relates to kinetic energy storage devices. More specifically, the invention is directed to such devices capable of translating electrical energy into the momentum of a rotating, magnetically suspended rotor element for storage during an indefinite period of time and translating this stored kinetic energy back into an electrical form with minimal losses.
2. Background
Devices for storing energy have been known for some time. The most common type of energy storage device is the lead-acid battery. Lead-acid batteries are known to have a limited amount of storage capacity, wear out after a relatively short period of time, contain numerous toxic metals and chemicals, fail without warning and are sensitive to high and low temperatures. Despite these problems, lead-acid batteries are the primary devices utilized to store energy for all types of uses, including automobiles and many everyday apparatuses. Lead-acid batteries, typically combined together in large arrays, also provide back-up power to those installations that need an uninterruptable supply of energy, including large computer installations, phone companies and many others. Electrically powered automobiles, which many are hoping will reduce pollution caused by internal combustion engine automobiles, require batteries to store energy for operation. To-date, however, the batteries available for automobiles are too costly, take too long to recharge, do not provide enough range, take up too much room and weigh too much.
Even the sophisticated batteries used in the space program, such as in satellites and space stations, have significant limitations due to the fact that the solar panels on the satellite or space station can only generate electricity for operation when the solar panels are located in the direct sunlight. When the satellite or space station passes through the Earth's shadow, it must rely on the energy stored in onboard batteries for its operation. As a result, these batteries are subject to frequent charge/discharge cycles that tend to wear out the batteries after a relatively short period of time. For satellites, the life span of these batteries can be the limiting factor for the life of the satellite itself. Due to the limitations and problems with presently available batteries, many people are looking into developing new technologies for energy storage. One such technology pertains to the use of flywheels.
For centuries, the venerable potter's wheel has demonstrated the capacity of a flywheel to effectively store energy in a kinetic form as a function of numerous variables, including rotor mass, diameter, and maximum rate of rotation, to name a few. The basic concept for modern flywheel energy storage devices is that electricity is fed into the battery to power a motor which accelerates the flywheel to high rotating speed by riding on magnetic bearings inside a vacuum container. The lack of resistance in the container (or housing) allows the flywheel to spin nearly indefinitely after the power input is cutoff. When needed, electricity is drawn out of the battery, with the motor functioning as a generator, causing the flywheel to slow down. Flywheel energy storage devices are not affected by the discharge/charge cycle problem of other batteries and do not have their high and low temperature limitations. In addition, it is generally known that flywheel devices can accept much higher levels of energy storage per pound and are not likely to have the limited life of presently available batteries.
Energy storage capacity in a flywheel device is typically increased by one of two ways, either with additional rotor mass or higher rates of speed. The mass affects energy storage on a one-to-one relationship, whereas speed has squaring effect on the energy storage. Materials available in the past, such as stone and metals, have been better suited to increases in rotor mass rather than speed. However, while a five-fold increase in rotor mass yields an equal five-fold increase in energy storage, a five-fold increase in rotor speed produces twenty-five times the energy storage capacity. As a result, many current flywheel development efforts focus on materials and designs which allow improvements in peak rotor velocity and attempt to counter the enormous centrifugal forces exerted on the rotor structure at extreme speeds. Since these radial forces also increase as the square of rotational speed, the properties of low density and high specific strength have made composite fibers (glass or carbon) bonded in a polymer matrix the materials of choice in high-speed flywheel projects during the past two decades.
Running on magnetic bearings, composite energy storage flywheels are expected to attain speeds in excess of 100,000 rpm and rim speeds of several thousand miles per hour. At such elevated speeds, energy densities up to ten times that of typical chemical battery systems can theoretically be achieved. Energy through-put efficiencies of 90-95% are expected, surpassing the 60-70% range that is typical of chemical batteries. As a result of their high energy densities and efficiencies, these super-speed energy storage flywheels are ideally suited to space applications, giving rise to the term “aerospace flywheel” for such devices. Space applications further benefit from the ability of these flywheels to serve a dual purpose, as integrated power and attitude control systems (“IPACS”), thereby not only replacing chemical batteries for energy storage, but also serving the role of the low-speed flywheels now used for gyroscopic orientation for many satellites.
At present, several technical challenges continue to plague aerospace flywheel development, challenges which have limited the world speed record for flywheel speed in a complete battery system to approximately 60,000 rpm. With some variation, the typical contemporary aerospace flywheel battery, shown in FIG. 1 (which is a NASA drawing) as 10, comprises a hoop-wound, composite filament rim 12 bound in a polymer matrix attached to a steel axle or shaft 14 via a metallic or composite hub structure 16. The steel shaft 14 is suspended, using magnetic bearing elements 18 (radial magnetic bearings) and 20 (axial magnetic bearing) inside a containment structure or housing 22 that is evacuated to minimize aerodynamic friction. A motor/generator 24 having high bi-directional efficiency and auxiliary bearings 26 are also utilized. The composite rim 12 is typically made of circumferentially wound carbon or glass fibers in a polymer matrix material, such as epoxy, to bind the fibers together. Although rim 12 designs of circumferentially wound composite fibers have excellent hoop strength, they have a very limited ability to withstand the enormous radial forces experienced during high-speed operation. The tendency of these rims 16 is to develop cracks between adjoining fibers from failure of the epoxy binder. Once these cracks occur, they elongate rapidly during charge/discharge cycling, eventually resulting in complete rim destruction.
Another limiting factor of the present flywheel designs is the cost, complexity, and limitations of the active bearing systems 18 and 20 employed in all current designs. Because of extreme frictional losses, simple mechanical bearings have been replaced with units consisting of a myriad of electronic and electrical devices designed to suspend the rotor mass, as well as damp out dynamic loads. A series of sensors monitor precise shaft 14 location numerous times per revolution, feed this data to a computer for evaluation, which in turn activates electromagnets 18 and 20 located along the rotor shaft 14 to correct fluctuations. Overall energy storage efficiency is reduced due to power requirements of the bearing systems. As with all mission-critical, failure-prone systems, redundancies are required, imposing a greater weight and cost burden on the flywheel unit 10. By their nature, computer controlled bearing systems are costly. In addition, extricating usable data from sensors and data processors, and applying that information to the electromagnetic bearings 18 and 20 in a timely manner at these speeds (and higher) is now recognized as a major, if not insurmountable hurdle, to overcome.
Prior art rotor structure (comprising the rim 12, shaft 14 and hub 16), as much as material selection, is crucial to performance at extreme speeds. As the interface between the magnetic bearings 18 and 20 and the rim 12 of a flywheel rotor, the shaft 14 and hub 16 unit must accommodate the transfer of extreme dynamic loads from the rotor's high-velocity rim 12 to the flywheel's housing 22 and support structure (not shown) via the magnetic bearings 18 and 20. In mobile applications, dynamic loads introduced from external sources must also be transferred back through the shaft 14 and hub 16 unit to the gyroscopically stabilized rim 12. While these forces vary from high-frequency oscillations to relatively long duration torsional loads, the cumulative effect will fatigue the shaft 14 and hub 16 materials, effectively reducing the lifespan of the rotor unit 10. The relatively close clearances required by modern magnetic bearings 18 and 20 tends to limit resilience in this location, leading some to favor a flexible hub 16 design in an attempt to isolate the rim-induced dynamic forces from the shaft 14 and hub 16. This added elasticity, however, can exacerbate the oscillations generated in the rim 12 at certain frequencies of rotation, increasing the likelihood of damage to the rim 12 itself.
With few exceptions, the rim structure of aerospace flywheels consists of continuous filaments of high-strength, light weight materials such as carbon fiber hoop-wound around a spool (i.e., the flywheel hub) and bonded together in a polymer matrix. Carbon fibers are reported to be four times stronger than the best steel, thereby allowing sixteen times more energy storage per pound than a similarly situated steel flywheel. The strength of these composite materials is extremely high inplane to the fibers, but is greatly reduced across the laminate thickness in a unidirectional matrix, as this strength is derived principally from the strength of the polymer binder, as well as the growth characteristics of the filament material under load. As a result, simple filament-wound flywheel rims have exceptional hoop strength, while their strength in the radial direction is severely limited. As speeds increase during the charge cycle, the hoop-wound filaments of a flywheel rim stretch to varying degrees, as a function of their distance from the axis of rotation. As discharge occurs and the rotor slows down, the filaments return to their unloaded state, contracting in length to their original dimensions. After repeated cycles, polymer binder fatigue can propagate microfracturing in the laminate structure, in turn leading to larger cracks and the eventual failure of the rotor rim. In order to maintain safety margins and ensure prolonged life of the flywheel unit, rotor speeds are presently limited by this factor. While some efforts have focused on alternative rim designs, either adding filaments in a radial direction or through other construction processes, few have achieved the benefit of higher speeds and efficiencies intended.
To date, every flywheel running on magnetic bearings has utilized a minimum of one, and often several, active magnetic bearings to achieve stable levitation. Such bearings are deemed “active” due to the electronically controlled servos, feedback sensors, and data processing equipment necessary to maintain the desired shaft position relative to the bearing elements. While non-controlled “passive” bearing elements have been successfully integrated into some high-speed flywheels, at least one axis of motion has always been controlled with an “active” element in each design.
While active bearing systems have matured and improved over time, numerous features make them ill suited for use in aerospace flywheel batteries. Constant parasitic power losses associated with the various electrical components of an active magnetic bearing tend to reduce the overall efficiency of the battery. Utilizing attractive magnetic forces and requiring relatively tight tolerances between the static and rotating portions of the bearing assembly, active bearings tend to be inherently unstable and sensitive to dynamic fluctuations. The position sensors and data processing equipment required by active magnetic bearings dramatically increase their overall cost compared to passive units of equal capacity. These additional components also make active bearings more prone to failure, often necessitating the further complexity and cost of high-speed, back-up mechanical bearings. In order to prevent rotor destruction in the event of even momentary magnetic bearing failures, these back-up bearings require precision components, resilience, and extraordinarily fast response times, adding even further to the cost and complexity of the prior art electromechanical battery unit.
As flywheel speeds have risen, their active bearings have been called upon to process data and carry out shaft-position corrections at ever increasing rates. With current shaft speeds exceeding 1000 revolutions per second, limitations imposed by bandwidth bottlenecks and servo response times are already common problems. What is needed, therefore, is an electromechanical battery that does not require active bearing elements, that has a composite rim which is capable of the higher speeds and able to withstand the resulting higher forces, and a passive magnetic bearing element to facilitate the high speeds necessary to obtain the capacity and efficiencies desired.